This invention relates to a power module (as will be called the xe2x80x9cintelligent power modulexe2x80x9d) which includes a self arc-extinguishing type semiconductor such as an insulated gate type bipolar transistor (as will be called the xe2x80x9cIGBTxe2x80x9d: Insulated Gate Bipolar Transistor) and which has a protecting function of the semiconductor.
The IGBT is determined in the magnitude of its permissible collector current by the magnitude of a gate voltage so that the collector current becomes the higher for the higher gate voltage. Moreover, the collector-emitter voltage of the IGBT rises the higher as the collector current becomes the higher.
In the collector-gate and the gate-emitter of the IGBT, as shown in FIG. 6, there exist parasitic capacitors Ccg and Cge. Therefore, the relation between a collector-emitter voltage VCE and a gate electrode VGE is expressed by:
VGE=xcex94VCExc3x97Ccg/(Ccg+Cge).
Here, Ccg less than  less than Cge,
xe2x80x83VGE=xcex94VCExc3x97Ccg/Cge.
In short, as the collector current increases, the collector-emitter voltage VCE increases so that the gate voltage VGE increases to increase the collector current and to accelerate the increase in the collector current. When an overcurrent flows through the IGBT, therefore, the collector current is abruptly increased by the aforementioned phenomenon to cause a problem that the IGBT has its turn-OFF allowable current exceeded.
Therefore, a method of preventing the gate voltage from increasing is exemplified by the prior art, as disclosed in Japanese Patent Laid-Open No. 262822/1990. FIG. 7 is a circuit diagram of an essential portion showing the prior art example.
In FIG. 7: reference numeral 27 designates an IGBT; numeral 21 a DC power source for applying a positive voltage to the gate when the IGBT 27 is turned ON; numeral 22 a DC power source for applying a negative voltage to the gate when the IGBT 27 is turned OFF; numeral 23 a transistor for applying a positive voltage to the gate of the IGBT 27 when turned ON; numeral 24 a transistor for applying a negative voltage to the gate of the IGBT 27 when turned ON; numeral 25 a gate-ON resistor for determining the gate voltage rising rate when the IGBT 27 is turned ON; numeral 26 a gate-OFF resistor for determining the gate voltage dropping rate when the IGBT 27 is turned OFF; numeral 28 a gate-emitter voltage limiting circuit of the IGBT 27; numeral 28A a transistor; numeral 28B a capacitor charged in advance with a voltage equal to that of the DC power source 21; numeral 28C a resistor; and numeral 28D a comparator.
When an xe2x80x9cHxe2x80x9d is inputted as a gate signal S1, as shown in FIG. 7, the transistor 23 is turned ON to apply a positive voltage to the gate of the IGBT 27 through the DC power source 21xe2x86x92the transistor 23xe2x86x92the gate-ON resistor 25. As a result, the IGBT 27 is turned ON. At this time, the turn-ON rate of the IGBT 27 is determined by a time constant of the gate-ON resistor 25 and the parasitic capacitor Cge of the IGBT 27.
When an xe2x80x9cLxe2x80x9d is inputted as the gate signal S1, on the other hand, the transistor 24 is turned ON so that a negative voltage is applied to the gate of the IGBT 27 via a route of the DC power source 22xe2x86x92the emitter of the IGBT 27xe2x86x92the gate of the IGBT 27xe2x86x92the gate-OFF resistor 26xe2x86x92the transistor 24. As a result, the IGBT 27 is turned OFF. At this time, the turn-OFF rate of the IGBT 27 is determined by a time constant of the gate-OFF resistor 26 and the parasitic capacitor Cge of the IGBT 27.
Here, the capacitor 28B is a capacitor having a sufficiently larger capacitance than that of the gate-emitter capacitor Cge of the IGBT 27. Moreover, the capacitor 28B is always charged with a voltage equal to that of the DC power source 21 by the leakage current through the emitter-collector of the transistor 28A.
For example, the collector current of the IGBT 27 becomes excessive so that this increase in the collector current leads to an increase in the collector-emitter voltage VCE and accordingly in the gate voltage VGE, as has been described hereinbefore. In this case, the voltage of the DC power source 21 and the gate voltage VGE are compared by the comparator 28D. When the gate voltage VGE exceeds the voltage of the DC power source 21, the output of the comparator 28D is set to the xe2x80x9cLxe2x80x9d level to turn ON the transistor 28A. At this time, the capacitor 28B is charged with the voltage of the DC power source 21 so that the gate voltage VGE is kept at the voltage of the DC power source 21. As a result, the gate voltage VGE does not exceed the voltage of the DC power source 21 so that the collector current is suppressed to such a current value as can be fed by the gate voltage determined by the DC power source 21.
Where the IGBT is applied to an inverter or the like, on the other hand, it is an important item to suppress the generation loss lowly. It is, therefore, necessary to drop the collector-emitter saturation voltage VCE (sat) of the IGBT. However, the collector-emitter saturation voltage VCE (sat) and the collector saturation current of the IGBT have the trade-off relation, as shown in FIG. 8. If the collector-emitter saturation voltage VCE (sat) is set low for the same gate voltage, therefore, there are increased the collector current (i.e., the collector saturation current) to be fed. Where the collector-emitter saturation voltage VCE (sat) is thus set low, the short-circuit resistance of the IGBT may be exceeded. In order to keep the short-circuit resistance, therefore, it is seriously difficult to drop the collector-emitter saturation voltage VCE (sat) to a predetermined or lower level.
In the prior art example thus far described, therefore, it is necessary for protecting the IGBT from the short-circuit current to set the collector-emitter saturation voltage VCE (sat) at the predetermined or higher level. This necessity makes it difficult to create an IGBT having a small loss and a large short-circuit resistance and accordingly to reduce the loss of the inverter device.
As the technique for protecting the IGBT against the short-circuit current, on the other hand, there is the prior art, as shown in FIG. 9, which is disclosed in Japanese Patent Laid-Open No. 79758/1992 or 139578/1996. In this prior art, there is adopted a method, by which the gate voltage is dropped, when a short-circuit current is detected, through a resistor Rg connected in series with a gate. In this prior art, after a gate voltage Vg is lowered, the gate-emitter voltage of the IGBT drops for the time period which is determined by the time constant of the gate-emitter capacitor Cge and the gate resistor Rg of the IGBT. As illustrated in FIG. 10, there is caused a delay Td1 till the gate voltage begins to drop and till the collector voltage begins to rise after the short-circuit current was detected. At this time, the short-circuit current continues to rise but then begins to drop with a delay Td2 after the collector voltage rose. Therefore, the short-circuit current does not drop before a delay of Td1+Td2 has passed after the short-circuit current was detected. Therefore, the short-circuit current has risen to such an extremely high value as may exceed the short-circuit resistance of the IGBT. Hence, there is a danger that the IGBT is broken.
This invention has been conceived to solve the aforementioned problems and has an object to provide a power module which is enabled to protect a self arc-extinguishing type semiconductor against a short-circuit current to flow through the collector-emitter of the semiconductor, even if the semiconductor is set with a low collector-emitter saturation voltage VCE (sat), by suppressing the peak value of the short-circuit current.
According to this invention, there is provided a power module comprising: a self arc-extinguishing type semiconductor having a current detecting emitter; and a short-circuit current suppressing circuit for turning OFF said self arc-extinguishing type semiconductor when a short-circuit current flows through the collector-emitter of said self arc-extinguishing type semiconductor, wherein said short-circuit current suppressing circuit includes: detection means for detecting said short-circuit current by detecting a current to flow through said current detecting emitter when the short-circuit current flows through the collector-emitter of said self arc-extinguishing type semiconductor; and a series member having a semiconductor switch and a capacitor and connected at its one terminal with the gate of said self arc-extinguishing type semiconductor and at its other terminal with the negative electrode of a power source connected at its positive electrode with the emitter of said self arc-extinguishing type semiconductor, and wherein said semiconductor switch is turned ON when said short-circuit current is detected by said detection means. When the short-circuit current flows through the collector-emitter of the IGBT, therefore, its peak value can be suppressed.
Moreover, said self arc-extinguishing type semiconductor is turned OFF a predetermined time after said semiconductor switch was ON. When the IGBT is turned OFF, therefore, the current to flow through the collector-emitter of the IGBT can be prevented from dropping abruptly.
Moreover, said short-circuit current suppressing circuit includes a resistor connected in parallel with the capacitor and having such a resistance that the gate-emitter voltage of said self arc-extinguishing type semiconductor when said self arc-extinguishing type semiconductor is turned OFF may be higher than the threshold voltage of said self arc-extinguishing type semiconductor. Therefore, the current to flow through the collector-emitter of the IGBT when this IGBT is turned OFF can be suppressed no more than the short-circuit resistance.
Moreover, said short-circuit current suppressing circuit includes a predetermined voltage diode connected in parallel with the capacitor and having such a resistance that the gate-emitter voltage of said self arc-extinguishing type semiconductor when said self arc-extinguishing type semiconductor is turned OFF may be higher than the threshold voltage of said self arc-extinguishing type semiconductor. Therefore, the current to flow through the collector-emitter of the IGBT when this IGBT is turned OFF can be suppressed no more than the short-circuit resistance.
Moreover, said series member further includes a resistor connected in series with the capacitor. When the semiconductor switch is turned ON, therefore, it is possible to prevent the current to flow through the collector-emitter of the IGBT from dropping abruptly.